The notification of offline users was removed from the mail server 13 years
ago (commit 729eb16b) without a lot of fanfare or explanation. Add an
option for notification of offline users (in addition to online users) for
those sysops that want that behavior.

Use the userdat is_user_online() function in place of the getnodedat() loop,
which also wasn't considering nodes in "logon" status as having a user online.